Tailoring the Magnetic Properties of Ferritic Alloys by HPT

Abstract

Industrial available FeSi, FeCo and FeNi alloys with an initial grain size of 20-50 m were subjected to Severe Plastic Deformation (SPD) up to strain levels where a saturation of the microstructural refinement is observed. For both SPD conditions, ambient temperature (293 K) and liquid nitrogen temperature (77 K), the microstructure of the severely deformed state is analysed by Back Scattered Electrons (BSE) micrographs captured in a SEM. Additionally, samples that were deformed at 77 K are examined in a Transmission Electron Microscope (TEM). The magnetic properties were characterised by means of SQUID-magnetography providing information about the magnetization behaviour of the material in the as-processed state. Depending on the SPD conditions the mean microstructural sizes in the steady state are about 100-150 nm and 30-80 nm at 293 K and 77 K, respectively. The small microstructural sizes influence significantly the magnetic properties of these ferritic alloys. The initial soft-magnetic behaviour of the coarse grained state shifts towards a hard-magnetic with decreasing crystallite size. For crystallite sizes smaller than ~80 nm the magnetic properties become again more soft-magnetic. The experiments show that very low coercitivity can be obtained by SPD if the grain size is smaller than ~50 nm.